1. Field of the Invention
The present invention relates generally to a fault-tolerant fiber optic backplane and, more particularly, to a fault-tolerant fiber optic backplane having a redundancy management unit for interconnecting local modules to external backplanes and their redundant modules.
2. Description of the Related Art
Fiber optic cable provides high speed data transfer, electrical isolation and immunity from electromagnetic interference. An application that would benefit from the use of fiber optic cable is the module interconnect (or "backplane" as it has been classically termed). A drawback of using fiber optic cable is that, to achieve high transmission speeds, it is necessary to synchronize the fiber optic transmitter and receiver. This requirement leads to point-to-point topologies, which are common in fiber optic networks. A backplane typically has a multi-drop connectivity, which allows each module on the backplane to directly access any other module on the backplane. Such connectivity is only achieved in point-to-point networks through fully connected topology, i.e., every module is connected to every other module. This type of connectivity is costly in terms of fiber optic transmitters and receivers.
In U.S. Pat. No. 4,870,637 issued to Follett et al. on Sep. 26, 1989, an optical backplane is described which uses a centralized switch that includes a switch block and a timing generation block. The centralized switch optically interconnects modules having fiber optic transmitters and receivers. To support high speed transmission, the fiber optic transmitters and receivers must be synchronized. For this purpose, a separate timing signal is provided to each module by the timing generation block. The addition of a separate timing signal, however, doubles the cost of a serial backplane, i.e., a backplane that transfers one bit at a time. Consequently, it is desirable to eliminate the requirement for a separate timing signal.
Many architectures have used time division multiplexing on internal electronic buses to interconnect modules and on external electronic buses to interconnect computers. Optical interconnects are usually designed with optical receiver clocks derived from a circuit which is phase locked with the transmitted optical signal. To maintain lock, the optical transmitter sends empty messages when real data is not available. Designers have not used time division multiplexing with optical buses due to the time penalty associated with reestablishing phase lock between the receiver/transmitter pairs when different transmitters begin to use the bus. Rather, token passing protocols, which carry less of a timing penalty, are used to provide optical interconnects. Token passing protocols, however, require more hardware and software to implement.
Conventional optical interconnects, such as the optical backplane disclosed in the Follett et al. patent, are not fault-tolerant. Conventionally, in electronic systems, fault tolerance is achieved by producing and maintaining redundant information that can be later analyzed to yield, even in the presence of errors in the redundant information, the correct original information. The degree of fault tolerance and, therefore, the reliability of the resulting system, depends on the amount of redundant information used. However, increasing redundancy adds cost both in material and performance.
One parameter of fault tolerance is the redundancy depth, e.g., dual, triplex and quad. U.S. Pat. No. 4,634,110 issued to Julich et al. on Jan. 6, 1987 describes a dual redundant fault detection and redundancy management system. Dual redundancy has the disadvantage of being unable to provide error masking, as may be provided with greater redundancy depth by incorporating voting circuitry. Instead, dual redundancy relies upon the passive failure of a faulty master unit.
Another parameter of fault tolerance is the level at which the redundant information is provided, e.g., gate, chip, module or subsystem. In the system disclosed in the Julich et al. patent, the redundant information is produced at the subsystem level, i.e., master unit. Thus, when a single component, e.g., a processor, of the subsystem fails the entire subsystem is disabled. This greatly increases the cost of failure and reduces system availability.
A third parameter of fault tolerance is synchronization. U.S. Pat. No. 4,497,059 issued to Smith on Jan. 29, 1985 and U.S. Pat. No. 4,665,522 issued to Lala et al. on May 12, 1987 disclose multi-channel redundant processing systems that are tightly synchronized, i.e., each channel is forced to execute identical instructions at exactly the same time. However, tightly synchronized systems are disadvantageous because they do not have the ability to simultaneously execute different instruction streams to increase reliability and performance.
U.S. Pat. No. 4,995,040 issued to Best et al. on Feb. 19, 1991 discloses an apparatus for management, comparison and correction of redundant digital data. This system uses a redundancy management unit that contains logic and control circuitry to implement fault tolerance algorithms. Similarly, U.S. Pat. No. 4,907,232 issued to Harper et al. on Mar. 6, 1990 discloses a fault-tolerant parallel processing system that includes network elements having logic and control circuitry to implement fault tolerance algorithms. Neither the Best et al. patent nor the Harper et al. patent address the requirements of connecting their respective redundancy management unit and network elements to fiber optic medium. Moreover, neither the Best et al. patent nor the Harper et al. patent address the vulnerability of the system to failure of their respective redundancy management unit and network elements. Making several replicated devices dependent on the correct operation of a single redundancy management unit as disclosed in the Best et al. patent, or on a single network element as disclosed in the Harper et al. patent, reduces the reliability and availability of the system.
Therefore, fiber optic backplanes have been proposed that are not fault-tolerant, and electronic fault-tolerant systems have been designed which do not address the needs of fiber optic interconnects. Further, the electronic fault-tolerant systems that have been proposed have limitations in system reliability and availability due to failure of a redundancy management unit or network element.